This invention relates to frequency converters employing a balanced bridge mixer and, more particularly, to a converter having plural output circuit branches connecting to nodes of the bridge mixer, wherein respective ones of the branches have respective filters with differing passband characteristics to pass respective signals of corresponding spectral characteristics while attenuating signals of other spectral characteristics.
Converter circuits are employed in transmitters and receivers of electric signals to provide an up-conversion of frequency and a down-conversion of frequency. The upconversion of frequency is obtained by mixing the signal of a local oscillator (LO) with a signal at intermediate frequency (IF) to obtain a radio frequency (RF) signal, and the down-conversion of frequency is obtained by mixing the RF signal with the LO signal to obtain an IF signal.
A common form of construction of such a converter circuit includes a balanced mixer. The balanced mixer has a bridge with an even number of arms, which are of nonlinear or switching elements, and which are arranged in a ring. Successive ones of the arms are interconnected by circuit nodes. One embodiment of the bridge employs a set of four matched field effect transistors (FETs) wherein the respective gate terminals of the transistors are driven by LO signals to provide alternative states of conduction and nonconduction within the transistors. The LO signal driving one pair of opposed transistors is of opposite phase to the LO signal driving the second pair of opposed transistors. The IF signal in an up-conversion, or the RF signal in a down-conversion, is applied to one pair of opposed nodes, typically by a transformer coupling. In both the case of up-conversion and down conversion, the output signal is obtained from the second pair of opposed nodes, typically by a transformer coupling.
In certain signal processing situations, it is desired to output any one of a plurality of signals wherein the signals are at different frequencies. This may occur, by way of example, wherein individual ones of the output signals are to be transmitted via different signal channels, or with different spectral characteristics and/or signal amplitudes. Alternatively, this may occur, by way of example, wherein received signals are to be processed via different signal channels which may differ in spectral passbands and/or signal power gain.
A problem arises in that, with the previously described construction of a converter, the converter has a single output channel. To accommodate the situation wherein the signal processor circuitry is to output signals in different frequency bands, it is the present practice to employ a plurality of converter circuits, particularly in the situation wherein the signals differ greatly in frequency so as to militate against a single wide band output channel. This is disadvantageous in requiring additional circuitry and additional consumption of direct current (DC) from a power supply. This disadvantage is most notable in the case of portable equipment such as may be carried by a spacecraft or in an electronic device, such as a communication device, carried by a person.